1. Field of the Invention
The present invention relates to an information storage medium on which information may be recorded/reproduced using a super resolution phenomenon and an apparatus for recording/reproducing information thereon, and more particularly, to an information storage medium to reduce reproduction low frequency noise and an apparatus to record and/or reproduce information thereon.
2. Description of the Related Art
An information storage medium is used in an optical pickup apparatus to record/reproduce information in a noncontact manner. The recording density of information needs to be increased with the development of industries. To this end, an information storage medium including a recording mark having a size that is smaller than a size formed by a resolving power (λ/4NA, where λ is the wavelength of a laser beam and NA is a numerical aperture of an objective lens) of a laser beam. That is, an information storage medium on which information may be recorded/reproduced using a super resolution phenomenon is being studied. In particular, a super resolution optical disk using a metallic oxide layer and a phase variation layer is being studied with great interest.
As an example of the super resolution optical disk using the metallic oxide layer and the phase variation layer, T. Kikukawa et al. APPLIED PHYSICS LETTERS, Vol. 81, No. 25, pp. 4697-4699, (16 Dec. 2002) describes a super resolution optical disk having a PtOx super resolution reproduction layer, wherein x is a positive number, and an Ag—In—Sb—Te reproduction layer. In the example, the PtOx super resolution reproduction layer is decomposed into Pt particles and oxygen by the application of the write power, a recording mark is formed, and an electron field coupling (EFC) in the vicinity of the Pt particles is a mechanism for super resolution reproduction.
As another example of the super resolution optical disk using the metallic oxide layer and the phase variation layer, J. Tominaga et al. NANOTECHNOLOGY, Vol. 15, pp. 411-415, (29 Jan. 2004) have written a paper on a super resolution optical disk in which a phase variation layer actually shows a super resolution phenomenon. In the paper, in a Ge—Sb—Te or an Ag—In—Sb—Te phase variation material, one crystal phase is changed into another crystal phase at a predetermined temperature, some of the optical characteristics in a laser beam spot are changed, and thus, super resolution reproduction is possible.
Meanwhile, those examples show an information storage medium having a phase variation layer shown in FIG. 1. Referring to FIG. 1, the information storage medium comprises a substrate 11, a first dielectric layer 13, a metallic oxide layer 15, a second dielectric layer 17, a phase variation layer 19, a third dielectric layer 21, and a cover layer 23, which are sequentially formed on the substrate 11. In this case, an incident beam focused by an objective lens OL is transmitted into the cover layer 23.
PtOx may be used as a metallic oxide that comprises the metallic oxide layer 15. When PtOx is used as the metallic oxide, PtOx is decomposed into Pt particles and oxygen when information is recorded, and thus, a recording mark 20 having an elliptical bubble structure is formed.
The recording mark 20 has both ends rounded. Thus, it is difficult to precisely set a boundary between a portion in which the recording mark 20 is formed and a portion in which the recording mark 20 is not formed, and it is recognized that when using an information recording/reproducing apparatus, the size of the recording mark 20 is nonuniform. Therefore, when information is reproduced on the recording mark 20, the level of a reproduction signal is irregular, and low frequency noise increases in a frequency spectrum.
In addition, when the size of the recording mark 20 is determined using an edge detection method of the recording mark 20, the irregularity of the signal level causes a reduction in jitter of the reproduction signal. Here, jitter refers to the standard deviation of a reference clock versus the length of a mark. Accordingly, jitter having an effective quality cannot be obtained from the super resolution information storage medium having the above structure.